Automotive electrical system

ABSTRACT

Disclosed herein are a variety of different electrical system topologies intended to mitigate the impact of large intermittent loads on a 12 volt vehicle power distribution system. In some embodiments the intermittent load is disconnected from the remainder of the system and the voltage supplied to this load is allowed to fluctuate. In other embodiments, the voltage to critical loads is regulated independently of the voltage supplied to the remainder of the system. The different topologies described can be grouped into three categories, each corresponding to a different solution technique. One approach is to regulate the voltage to the critical loads. A second approach is to isolate the intermittent load that causes the drop in system voltage. The third approach is to use a different type of alternator that has a faster response than the conventional Lundell wound field machine.

The present application claims priority from provisional applicationSer. No. 60/599,328, entitled “Automotive Electrical SystemConfiguration,” filed Aug. 6, 2004, which is commonly owned andincorporated herein by reference in its entirety.

The present application is related to non-provisional application, Ser.No. ______, entitled “Automotive Electrical System Configuration Using aTwo Bus Structure,” attorney docket IS01801AIC, filed Aug. 24, 2004,which is commonly owned and incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

This invention in general relates to automotive electrical systems and,more particularly, to configurations of automotive electrical powersystems adapted for use with high power loads.

BACKGROUND OF THE INVENTION

The 12 volt systems used in today's automobiles are required to supplyever increasing currents as the load on the system continues toincrease. This increase is due to a combination of increasing numbers ofelectronic devices, such as communication, entertainment, and telematicssystems, as well as the proliferation of electric powered auxiliarysystems to replace traditional hydraulic or mechanical powered systems.To reduce the amount of current required to supply these higher loads,it has been proposed that automobiles should adopt 42 volt electricalsystems. However, the automotive industry has been reluctant totransition to 42 volt electrical systems because of increased costs.Consequently, there is a strong demand to improve the performance of 12volt systems, thereby allowing higher electrical loads to operateeffectively with conventional vehicle electrical systems.

As an example, high current loads, such as electric power steering(EPS), cannot practically be used in larger vehicles, such as lighttrucks, with conventional vehicle electric systems. EPS in particularplaces a large demand on the electrical system because it draws a largecurrent at low vehicle speeds, which is where the most steering assistis required. However, at low vehicle speeds, e.g. in a parking lot, theengine is typically at or near idle and thus alternator current outputcapability is severely limited. As a result, the vehicle electricalsystem cannot supply the power needed by EPS without the 12 volt busexperiencing a temporary voltage dip. When this voltage dip occurs, avariety of objectionable performance is experienced from variouselectrical systems, for example dimming of the vehicle lights.Additionally, it is also likely that the required EPS current cannot besupplied, and thus the desired steering response will not occur.

A variety of solutions to the problem of supplying high current loads invehicle electrical system have been proposed. European patentapplication EP0533037A1, entitled “An Electrical System for a MotorVehicle, Including at Least One Supercap” describes a circuit andsupercapacitor arrangement that is connected across a load. The load isenergized initially from the supercapacitor. The amount of energy drawnfrom the supercapacitor is not optimized because the supercapacitor isconnected directly across the load, thereby limiting the voltage dropacross the supercapacitor. Also, there is no isolation of theload/supercapacitor circuit from the battery other than a simple diode,so the temporary power provided to the load is not entirely decoupledfrom the battery.

U.S. Pat. No. 5,914,542, entitled “Supercapacitor Charging” describes aDC power distribution system for a fighter aircraft. In the describedsystem, the battery is located remotely from the load. A supercapacitoris connected to the DC bus through a supercapacitor boost convertercombination close to the load. The supercapacitor is normallydisconnected from the bus, but when a load transient occurs, the batteryis disconnected from the load and the load is supplied from thesupercapacitor. This system is disadvantageous in that the energysupplied to the load is limited solely to that in the supercapacitor.

The present invention attempts to minimize the above-mentioned drawbacksand proposes a system that solves or at least minimizes the problems ofthe prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an automotive electrical system in which a criticalload is powered by a boost converter to minimize the effects of voltagedip on the main bus caused by a high current transient from anotherload;

FIG. 2 illustrates a plot of inductor current for headlights;

FIG. 3 illustrates an automotive electrical system in which a highcurrent load is isolated from the main electrical bus by a bidirectionalDC-DC converter and in which a supercapacitor is provided to supply highcurrent transients through a second bidirectional DC-DC converter;

FIG. 4 illustrates a variation of the circuit of FIG. 3 in which the twoDC-DC converters are replaced with a single tri-directional DC-DCconverter;

FIG. 5 illustrates another embodiment where two bidirectional DC-DCconverters are integrated into a single package;

FIG. 6 illustrates another variation of the circuit of FIG. 3 in whichthe position of the high current transient load and the other electricalloads have been interchanged;

FIG. 7 illustrates an automotive electrical system in which a highcurrent transient load is isolated from the remaining electrical loads;and

FIG. 8 illustrates still another automotive electrical system in which aconventional wound field alternator is replaced with a switchedreluctance generator.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

What is described are electrical system topologies intended to mitigatethe impact of large intermittent loads on a 12 volt vehicle powerdistribution system. In some embodiments, the intermittent load isdisconnected from the remainder of the system and the voltage suppliedto this load is allowed to fluctuate. In other embodiments, the voltageto critical loads (e.g., the headlights) is regulated independently ofthe voltage supplied to the remainder of the system. The differenttopologies described can be grouped into categories, each correspondingto a different solution technique.

One approach is to regulate the voltage to the critical loads. Asolution in this mode is to provide a separate boost converter forcritical loads, as illustrated in FIG. 1. A second approach is toisolate the intermittent load (e.g., EPS) that causes the drop in systemvoltage. These solutions typically involve multi-directional DC/DCconverters and are illustrated in FIGS. 3-7. The third approach is touse a different type of alternator that has a faster response than theconventional Lundell wound field machine. This approach is illustratedin FIG. 8.

Now, turning to the drawings, FIG. 1 illustrates an automotiveelectrical power system according to certain teachings of the presentinvention. In FIG. 1, the voltage to the critical loads, e.g.,headlights 108, is regulated using a separate converter (i.e., boostconverter 111) while the remainder of the 12 volt system suffers fromvoltage dips because the alternator does not respond fast enough.

Vehicle main power bus 101 is supplied with a voltage in the range of 9to 16 volts DC. This energy is supplied from alternator 102, which, inthis embodiment is a typical wound field alternator as is known to thoseskilled in the art. Current flows from alternator 102 to bus 101 throughrectifier bank 103. The rectifier bank 103 may comprise one or morediodes, as is typical, or may comprise controlled switched rectifierssuch as transistors, e.g., field effect transistors (FETs), or siliconcontrolled rectifiers (SCRs). Additionally, both alternator 102 andrectifier bank 103 may be of single phase or multi-phase form.

Storage battery 104 is also connected to bus 101. In many automotiveapplications, this battery is a conventional lead acid battery, althoughvarious other battery types may also be used. During normal vehicleoperation, battery 104 does not supply steady state energy to bus 101and the loads connected thereto. The electrical energy required fornormal vehicle operation is provided by alternator 102, assuming thatthe capacity of the alternator is sufficient to provide the requiredpower. If this power cannot be supplied by the alternator, power isdrawn from the battery. In addition, battery 104 is available to providepower to the various electrical loads when the vehicle is not inoperation.

During normal vehicle operation, battery 104 is charged from bus 101.Battery charging current may be left uncontrolled, as is typical, orvoltage regulator 105 may be configured to regulate the charge currentand voltage supplied to the battery. However, in normal operation,voltage regulator 105 is operative to keep the voltage of the bus 101 ata nearly constant value. This is necessary because the output voltage ofalternator 102 varies with engine speed and the electrical loadconnected to the bus 101. Design of various voltage regulator circuitsis well known to those skilled in the art, and thus is not addressed indetail here.

A variety of electrical loads are supplied with power by bus 101. Theseinclude miscellaneous loads 106, which are typical electrically powereddevices in automobiles, such as radios, interior lighting, HVAC blowers,etc. An additional load in the illustrated example is an electric powersteering (EPS) system 107, which, as discussed above is a highpower/high current load. Still further electrical loads, in this casecritical loads such as headlights 108 and other critical loads 109 arealso connected to bus 101, albeit indirectly by smart junction box 110and boost converter 111.

Smart junction box 110 has a microcontroller that controls the turn onand off of the lights using a high side switch. This is distinguishedfrom a conventional junction box, which has no ability to control theloads receiving power therefrom. Having a smart junction box capable ofload control is advantageous because the current inrush into the lightswhen first turned on can be controlled, thereby extending the lifetimeof the lights. Most of the control of auxiliary loads (excluding highcurrent loads such as the starter and EPS) is consolidated into smartjunction box 110 rather than split into two or three different loadcontrol boxes.

In the embodiment of FIG. 1, the critical loads are supplied throughboost converter 111 which regulates the voltage to this load. When oneof the loads connected directly to bus 101 draws a large amount ofcurrent, causing a voltage dip on bus 101, the boost converter 111boosts the voltage supplied to critical load 108 to maintain arelatively constant voltage in the 13.5-16 volt range to the criticalloads such as the headlights. This prevents undesirable side effects ofthe high current loads, such as dimming of the headlights. Additionally,this topology is advantageous in that it requires relatively minorchanges to existing vehicle electrical systems. Other than the additionof smart junction box 110 and boost converter 111, the remainder of theelectrical system remains unchanged. Additionally, some currentproduction vehicles already include a smart junction box, so only boostconverter 111 need be added.

In one embodiment, the boost converter 111 may operate in a current modecontrol loop, as explained in “An Accurate and Practical Small SignalModel for Current Mode Control”, from Ridley Engineering Inc(www.ridleyengineering.comn). A boost control algorithm 130 can be usedto control a switch 132 in the boost converter 111. A sense resistor maybe used to read the current (I_(sense)) through the switch 132 and todetermine when the switch turns off. Additionally, other methods ofsensing current may include reading the inductor current directly. Notethat when the switch is turned on, the switch current and inductorcurrent are the same.

FIG. 2 shows a plot of inductor current versus time for two cases: onewhere both lamps (of the headlights) are operating properly and anothercase where one of the lamps is open and no current flow through thislamp. Consequently, as shown in FIG. 2, the average current through theinductor drops down, approximately 50% of the two-lamp case. Themalfunction condition is determined by comparing operation of thecircuit when both lamps are operating to that when only one lampoperates. The peak inductor current is directly related to the averageinductor current and is used in this case to determine if a lamp isopen. When the switch is turned off, the peak current is sampled and iscompared to the expected current for the two-lamp case (both lamps on).If it is approximately 50% of its expected value, an open lamp conditionis indicated. Clearly, if the current is almost zero, both lamps areopen and a fault condition is also indicated.

In some cases, each lamp may be powered by separate boost circuits,instead of one combined boost circuit. In this case, an open lamp isdetermined by comparing the peak current in each boost circuit. It isexpected that the currents in each boost circuit will be approximatelythe same and will not be close to zero amps, in which case both lampsare open circuit.

The boost circuit 111 may be disabled in certain cases where boostoperation is not desired even though the boost output voltage (V_(out))is less than the desired value. One of these cases is where daytimerunning lights are energized. It is desirable to reduce the voltage tothe headlights when in a daytime running mode to prolong the bulblifetime. In this case, the circuit of the boost converter 111 isdisabled and the voltage to the headlights 108 is less than the batteryvoltage as a result of smart junction box 110 operation. Another casewhere the boost is disabled is when the lights are first turned on andthe inrush current to the lamps is controlled by the smart junction box110. A current inrush occurs because the lamp resistance is low whenfirst turned on and increases to its final resistance only after thelamp in the headlights 108 heats up—100 ms is a typical time. In thiscase, the circuit of the boost converter 111 is temporarily disabled bythe smart junction box 110 until the inrush current has reached itsfinal value. At this time, the boost converter 111 is enabled and theoutput voltage is boosted, if needed.

Another embodiment is disclosed in FIG. 3. In this topology, EPS 107 andbattery 104 are isolated from the remainder of the system, to preventvoltage dips caused by high current transients of EPS 107 from affectingthe rest of the system. As in the embodiment of FIG. 1, electricalenergy is supplied by an engine driven alternator 102, which is atypical wound field machine. The AC voltage produced by the alternatoris rectified by rectifier bank 103 thus supplying a DC voltage of about13.5 volts to main bus 101. Voltage regulator 105 serves to maintain thevoltage of main bus 101 at a relatively constant value. A variety ofmiscellaneous loads 106 are connected directly to main bus 101, as arecritical loads such as the headlights 108 and other critical loads 109,both of which are connected to the main bus 101 by smart junction box110.

In the embodiment of FIG. 3, the high current load, e.g., EPS 107, isconnected directly to the battery 104 by secondary bus 101 a. Secondarybus 101 a and the devices connected thereto are isolated from theremainder of the system by a bidirectional DC-DC converter 113 a. Aswith the boost converter 111 of FIG. 1, the bidirectional DC-DCconverters 113 a and 113 b may take a variety of forms known to thoseskilled in the art. Typically these converters will be from the categoryof bidirectional buck-boost converters, of which the illustrated exampleis one of the most simple and typical types. The function ofbidirectional DC-DC converter 113 a is to control the flow of currentbetween bus 101 and secondary bus 101 a. This can be done by using abattery charge algorithm 120, explained further below. Depending on thevoltage levels of buses 101 and 101 a, the converter 113 a will functionas either a boost converter or a buck converter as required.

During normal operation, the DC-DC converter 113 a connects the battery104 and EPS 107 to the rest of the electrical system. This allowsbattery 104 to be charged and provides normal operating currents to EPS107. When a large EPS transient occurs, the current for the remainder ofthe system is supplied by alternator 102 and supercapacitor 112, whichdelivers electrical energy to main bus 101 through bidirectional DC-DCconverter 113 b. Supercapacitor 112 is essentially a relatively highcapacitance capacitor based on a hybrid of capacitor and batterytechnology supercapacitors. Supercapacitors, also known asultracapacitors, are generally known to those skilled in the art, andtherefore details of these devices are not repeated here.

In one embodiment, a controller for the bidirectional DC-DC converter113 a executes a battery charge algorithm 120, as described in “Chargingthe Lead Acid Battery” by Isidor Buchman (www.batteryuniversity.com).This algorithm charges the battery 104 at constant current when deeplydischarged and charges at constant voltage otherwise. In most automotiveapplications, a constant charging voltage is applied to the battery 104,for instance 14.5V and it depends on the ambient temperature.

A second bidirectional DC-DC converter 113 b works in conjunction withvoltage regulator 105 to stabilize the voltage of main bus 101 at 13.5volts, as explained further below. Additionally, the first bidirectionalDC-DC converter 113 a provides a degree of isolation between highcurrent load EPS 107 and the rest of the electrical system, whichfurther contributes to the voltage stability of main bus 101. As aresult, the boost converter 111 that supplied the power to the criticalloads in the embodiment of FIG. 1 is not required. The configuration ofFIG. 3 is advantageous where the large currents are needed by theintermittent loads and loads other than lights, e.g., loads 109 and 106,need a regulated voltage supply. As the second bidirectional DC-DCconverter 113 b and the alternator 102 respond to the increased need forcurrent, the first bidirectional DC-DC converter 113 a can redirectavailable energy to the secondary bus 101 a, thereby reducing the impacton the battery voltage. Once the alternator 102 is capable of supplyingall of the power needed for bus 101 and secondary bus 101 a, the firstbidirectional DC-DC converter 113 a acts as a pass-through and buck orboost operation is not utilized. The loads 106 and 110 are shownconnected to the alternator 102 but they may also be connected to thesupercapacitor 112. It might also be advantageous to separate thedifferent loads 106 and 110 so they do not connect to the same side ofthe converter 113 b.

In one embodiment, a controller for the second bidirectional DC-DCconverter 113 b executes an alternator and supercapacitor algorithm 122to maintain the bus 101 at a constant voltage as the primary constraint,for instance 13.5V. This is achieved through a combination of thealternator output current (I_(alt)) and the current drawn from orsupplied to the supercapacitor 112 (I_(uc)). As an addititionalconstraint, the supercapacitor voltage (V_(uc)) is maintained at a fixedvoltage, for instance 20V, by managing the current supplied to thesupercapacitor 112 when bus 101 voltage exceeds the set value. In oneembodiment, using the alternator current (I_(alt)), alternator voltage(V_(alt)), supercapacitor current (I_(uc)), and supercapacitor voltage(V_(uc)), the following psuedo-code may be used to achieve both of theseconstraints: If (V_(alt) > = 13.5V) If (I_(uc) > = 0) force I_(uc) lesspositive else If ((V_(uc) < 20V) AND (I_(uc) < I_(uc) _(—) max) forceI_(uc) more negative else force I_(uc) less negative force I_(alt) lesspositive using PI controller (note 1) endif /* V_(uc), I_(uc) */ endif/* I_(uc) */ elseif (V_(alt) < 13.5V) If (I_(uc) >= 0) force I_(uc) morepositive positive force I_(alt) more positive positive using PIcontroller and I_(uc) component (note 2) else force I_(uc) less negativeforce I_(alt) more positive positive using PI controller endif /* I_(uc)*/ endif /* 13.5V */

Note 1: A proportional-integral (PI) controller may be used to regulatea field voltage (V_(f)) as a means to force the voltage of thealternator (V_(alt)) to its desired value (13.5V in this example). Thiscan be done by comparing alternator voltage (V_(alt)) to 13.5V andsetting the value of the field voltage (V_(f)). In one embodiment,relation (1) may be used:V _(f) =K _(P)·(13.5−V _(alt))+K _(I)·∫(13.5−V _(alt))  (1)

where V_(f) is the field voltage, K_(P) and K_(I) are the proportionaland integral gains of a well-known PI (proportional-integral)controller, and V_(alt) is the alternator voltage.

Note 2: In this case, the PI controller is augmented by a componentwhich depends on the current of the supercapacitor (I_(uc)) usingrelation (2):V _(f) =K _(P)·(13.5−V _(alt))+K _(I)·∫(13.5−V _(alt))+K _(uc) ·I_(uc)  (2)

where V_(f) is the field voltage, K_(P) and K_(I) and K_(uc) are theproportional and integral gains of a well-known PI(proportional-integral) controller, and I_(uc) is the current of thesupercapacitor 112.

A typical method to force the current of the supercapacitor (I_(uc)) toits desired value is to use relation (3):V _(control) =K _(uc) ·I _(uc)  (3)

In this case, V_(control) is used to set the duty cycle on the switchesin the second bidirectional DC-DC converter 113 b.

FIG. 4 illustrates a variation of the embodiment of FIG. 3. As in theembodiment of FIG. 3, EPS 107 and battery 104 are isolated from theremainder of the system. The two bidirectional DC-DC converters 113 aand 113 b of FIG. 3 have been replaced with one tri-directionalconverter 114. Tri-directional converter 114 is basically a combinationof converters 113 a and 113 b. The power flow into and out of each ofthe three terminals is managed depending on the voltage of each terminaland the priority of each bus. A terminal which has highest priority willhave its voltage maintained at the regulated point at all times to thedetriment of the other terminal voltages. In operation, thetri-directional converter 114 is capable of transferring energy to orfrom any combination of main bus 101, secondary bus 101 a, andsupercapacitor 112.

One difference between the embodiment of FIG. 4 and the embodiment ofFIG. 3 is that voltage regulator 105 is connected to tri-directionalconverter 114, and the voltage set point of the regulator is not fixedto 13.2 volts output. In the embodiment of FIG. 4, the regulator is anintegral part of the electrical system and operates under the control oftri-directional converter 114. Properly designed, the topologyillustrated in FIG. 4 will have improved system response over thetopology of FIG. 3 because the system operation is centrally controlled.Enhanced system response could also be obtained by implementing currentsensor 114 a, which allows the control circuit of tri-directionalconverter 114 to respond to the output current of alternator 102.Additionally, one could also implement the basic topology of FIG. 3while replacing bidirectional converters 113 a and 113 b withtri-directional converter 114 of FIG. 4.

FIG. 5 illustrates another embodiment where the bidirectional converters113 a and 113 b are integrated into the same package. Here, the topologyincludes an integrated bidirectional converter 124 and positioned toallow the EPS 107 and the battery 104 to be isolated from the remainderof the system. A controller for the integrated bidirectional converter124 executes an algorithm 126. In this case, each portion of theintegrated bidirectional converter 124 is controlled by the samealgorithms 120 and 122 discussed above in relation to FIG. 3.Accordingly, the algorithm 126 includes a battery charge algorithm 120component and an alternator and supercapacitor algorithm 122 componentdetailed above. The operation of the integrated bidirectional converter124 is capable of transferring energy to and from the main bus 101 andthe supercapacitor 112.

FIG. 6 illustrates another variation of the topology illustrated in FIG.3. The topology of FIG. 6 differs from FIG. 3 in that the high transientcurrent load, i.e., EPS 107, and the remainder of the circuit have beenswapped. Thus miscellaneous loads 106, headlights 108 and other criticalloads 109, along with battery 104, are connected to secondary bus 101 a.As in the other cases, headlights 108 and other critical loads 109 areconnected to secondary bus 101 a by smart junction box 110. Alternator102 powers main bus 101, to which EPS 107 is connected.

The bidirectional DC-DC converters 113 a and 113 b are each controlledin the same manner described above in FIG. 3 by the battery chargealgorithm 120 and the alternator and supercapacitor algorithm 122. Whenthe EPS experiences a high current transient, it is disconnected fromthe battery via the bidirectional DC-DC converter 113 a. During thishigh current transient, supercapacitor 112 provides excess current toEPS 107 through bidirectional DC-DC converter 113 b. During normaloperation, bidirectional DC-DC converter 113 b is operative to chargesupercapacitor 112 from main bus 101, and bidirectional DC-DC converter113 a operates to energize secondary bus 101 a which charges battery 104and powers headlights 108, critical loads 109, and miscellaneous loads106.

Still another automotive electrical system topology for providing astable bus voltage to critical loads in the presence of high currentloads is illustrated in FIG. 7. In this arrangement, EPS 117 andalternator 102 are isolated from the remainder of the system. Thecircuit depicted in FIG. 7 is essentially an amalgamation of the circuitof FIG. 1 with the circuits of FIGS. 3-6. As with the circuit of FIG. 1,headlights 108 are powered by a boost converter 111, although dependingon the parameters of the rest of the system (time constants, etc.),boost converter 111 may not be required. The voltage on main bus 101 iscontrolled by bidirectional DC-DC converter 113, similar to theembodiments disclosed in FIGS. 3-6.

The high current load, EPS 107, is connected across the alternator, and,more specifically, across rectifier bank 103. Bidirectional DC-DCconverter 113 isolates EPS 107 from the remainder of the system. Thus,during a large EPS current transient, the voltage across the alternatordrops and, as a result, EPS 107 is disconnected from the remainder ofthe circuit. Consequently, the voltage drop across the battery ismanaged with priority given to regulating the system voltage.

One advantage of the topology illustrated in FIG. 7 is that EPS 107 neednot be designed for a 13.2 volt system bus for all operating points. Itis advantageous to increase the voltage to the EPS under high load(i.e., high EPS motor speed) conditions. To optimize this embodiment,EPS 107 needs to communicate to the alternator 102 when the load willincrease or decrease to minimize the magnitude of alternator voltageswings due to increased EPS load. This takes the form of a typicalfeed-forward control circuit illustrated schematically by connection 117in FIG. 7. This arrangement provides a major improvement in voltageregulation. This arrangement also provides the advantage of eliminatingthe supercapacitor or other energy storage device.

As mentioned above, in FIG. 7, the bidirectional DC-DC converter 113acts as an isolator to disconnect EPS 107 from the remainder of thesystem if the EPS 107 current cannot be supplied by thealternator/battery combination while keeping the battery voltage withinthe desired range. In this way, EPS 107 cannot cause the secondary bus101 a voltage to vary outside the desired range, say 12.8V to 14.1V.This change in the secondary bus 101 a voltage must occur slowly enoughthat the change in light brightness of the headlights 108 is notperceivable to the driver. This circuit differs from that of FIG. 6 inthat the supercapacitor is removed, thereby forcing a greater variationin voltage onto the bus 101.

The EPS voltage will undergo large magnitude changes if disconnectedfrom the battery 104 because there is no energy source/sink to provideshort term storage, indicated in FIG. 7 as bus 101. A range of 10-20V isindicated in FIG. 7 but this is only a representative range, and actualcircuit values will depend on design constraints in the bidirectionalDC-DC converter 113, the alternator 102 and the EPS 107. In most circuittopologies, this energy storage is provided by a battery orsupercapacitor. In cases where large power is needed by EPS 107, this isadvantageous because the alternator output voltage may be increased wellabove the battery voltage, thereby decreasing alternator current and EPScurrent. During this time, current is supplied to the battery and loadcircuits by the bidirectional converter so the battery is notdischarged.

To reduce the voltage swings on the output of the alternator 102, afeedforward signal 117 is used between the EPS 107 and the voltageregulator 105. The field voltage may be regulated according to relation(4):V _(f) =K _(P)·(13.5−V _(alt))+K _(I)·∫(13.5−V _(alt))+K _(FF) ·_(eps)  (4)

where V_(f) is the field voltage, K_(P) and K_(I) are the proportionaland integral gains of a well-known PI (proportional-integral)controller, V_(alt) is the alternator voltage, K_(FF) is the feedforward gain and l_(eps) is the EPS current. Selection of an appropriatefeed forward gain for the battery current will lead to an increase ordecrease in field voltage before the output voltage changes. Designtechniques for these controllers are generally known to those skilled inthe art, and may also be found in “Computer Controlled Systems: Theoryand Design”, by Astrom/Wittenmark, 1990, pp. 150-151 (which isincorporated by reference). The feedforward gain, K_(ff), may be variedas a function of field current if the alternator rotor is in saturation.When the rotor is in saturation, an increase in field current results ina smaller increase in back emf and a correspondingly smaller increase inbattery charging current, i.e., diminishing returns. The saturationphenomenon is explained in “Electric Machinery”, by Fitzgerald et. al.,1983, p. 176-178, which is incorporated by reference. When the machineis saturated, a plot of field current versus open circuit voltage showsa deviation from a straight line. As the field current increases andsaturation begins, the constant slope reduces as the output voltageincrease in less than the field current increase. Ideally, K_(ff) ismodified so that the product of K_(ff) and the inverse of the opencircuit curve slope is a constant. In effect, K_(ff) increases at theonset of saturation and continues to increase as the amount ofsaturation increases.

A final embodiment is illustrated in FIG. 8. This embodiment features agenerator with a faster response time to eliminate the need forisolating EPS 107 from the remainder of the system. This circuit istopologically similar to the circuit of FIG. 1, except the traditionalalternator 102 and rectifier bridge 103 have been replaced by a switchedreluctance generator 115 and switch reluctance controller 116. Switchedreluctance generators and their controllers are known to those skilledin the art, and thus details of their construction and configuration arenot repeated here. A switched reluctance generator is preferable to aconventional wound field alternator because of the enhanced transientresponse of the switched reluctance machine. The response of a typicalLundell wound field alternator is characterized by a time constant ofabout 0.2 seconds, which is a result of the relatively large inductanceof the field winding. In contrast, a switched reluctance generator has atime constant on the order of 25 milliseconds.

Also included in FIG. 8 is a boost converter 111 for headlights 108,although in a well-designed system boost converter 111 would not berequired because of the enhanced transient response of switchedreluctance generator over conventional wound field alternators. It isalso noted that the circuit disclosed in FIG. 8 includes feed forwardcontrol from EPS 107 to switched reluctance generator controller 116,which further helps to enhance the response time of the system to EPShigh current transients. Furthermore, a switched reluctance machine mayalso be advantageously employed with any of the automobile electricalsystem topologies disclosed herein.

A further enhancement of FIG. 8 includes a voltage sense windingconnected across the critical loads that is used as the voltage feedbacksignal for the switched reluctance generator controller. The sensedvoltage can be either the supplied voltage as shown in feedback loop 118a or both the positive sensed voltage 118 a and the return voltage asshown in feedback loop 118 b. This is a practical control method becauseof the reduced response time of the switched reluctance generator ascompared to the response of the wound field machine. This additionimproves the voltage regulation across the critical loads because thevoltage drop due to the bus interconnect resistance and inductance isbypassed. In contrast, today's wound field machine uses the rectifier103 output as the feedback reference point. The optimal feedbacksituation is to feedback both the positive and ground return voltageacross the load, i.e., both feedback loops 118 a and 118 b, to mitigatethe effect of the positive and negative bus leads.

In FIG. 8, the faster time constant of the switch reluctance generator115 eliminates the need for a supercapacitor to provide a short termcurrent supply. However, a feedforward signal 117 from the EPS 107 maystill be used here to reduce the transient voltage change when a largeEPS current is drawn. The switch reluctance control algorithm may bewritten according to relation (5):V _(control) =K _(P)·(13.5−V _(alt))+K _(I)·∫(13.5−V _(alt))+K _(FF) ·I_(eps)  (5)

The operation of the switch reluctance generator is described in“Brushless PM and Reluctance Motor Drives”, by T. J. E. Miller,published by Clarendon Press, Oxford, 1989. As described in this text,the ideal current waveform is a square wave and the role of V_(control)is to modulate the level of this current to vary the amount of currentavailable at the output of the converter.

It should be understood that the inventive concepts disclosed herein arecapable of many modifications, combinations and subcombinations. Forexample, the boost converter described with reference to FIG. 1 mayadvantageously be combined with other embodiments. Similarly, the feedforward control circuit of FIG. 8 may also be advantageously combinedwith other embodiments. Still further combinations are also possible. Tothe extent such permutations fall within the scope of the appendedclaims and their equivalents, they are intended to be covered by thispatents.

1. An automotive electrical system comprising: a main bus; a batteryconnected to the main bus; an alternator connected to the main busthrough a rectifier bank and controlled by a voltage regulator; aplurality of loads connected to the main bus; at least one critical loadwherein the critical load is powered by an electronic power converterconfigured to maintain a relatively constant voltage supplied to thecritical load in the presence of a voltage dip on the main bus as aresult of a high current transient caused by one or more of theplurality of loads connected to the main bus.
 2. The automotiveelectrical system of claim 1 further comprising a smart junction boxconnected between the main bus and the electronic power converter,wherein the smart junction box includes a microcontroller forcontrolling one or more loads connected thereto.
 3. The automotiveelectrical system of claim 1 wherein the alternator is a switchedreluctance generator and wherein the voltage regulator and rectifierbank are an integrated switched reluctance controller.
 4. The automotiveelectrical system of claim 3 further comprising a feedback pathconnecting the voltage supplied to the critical load to the switchedreluctance controller.
 5. The automotive electrical system of claim 2wherein the alternator is a switched reluctance generator and whereinthe voltage regulator and rectifier bank are an integrated switchedreluctance controller.
 6. The automotive electrical system of claim 5further comprising a feedback path connecting the voltage supplied tothe critical load to the switched reluctance controller.
 7. Theautomotive electrical system of claim 1 wherein the electronic powerconverter is a boost converter.
 8. The automotive electrical system ofclaim 2 wherein the electronic power converter is a boost converter. 9.The automotive electrical system of claim 3 wherein the electronic powerconverter is a boost converter.
 10. The automotive electrical system ofclaim 4 wherein the electronic power converter is a boost converter. 11.The automotive electrical system of claim 5 wherein the electronic powerconverter is a boost converter.
 12. The automotive electrical system ofclaim 6 wherein the electronic power converter is a boost converter. 13.The automotive electrical system of claim 1 further comprising a feedforward control circuit for controlling the voltage regulator inresponse to a current required by the high current load.
 14. Theautomotive electrical system of claim 2 further comprising a feedforward control circuit for controlling the voltage regulator inresponse to a current required by the high current load.
 15. Theautomotive electrical system of claim 7 further comprising a feedforward control circuit for controlling the voltage regulator inresponse to a current required by the high current load.
 16. Theautomotive electrical system of claim 8 further comprising a feedforward control circuit for controlling the voltage regulator inresponse to a current required by the high current load.
 17. A method ofdistributing electrical power in an automotive electrical systemcomprising: generating electrical energy; delivering the electricalenergy to a main bus; storing the electrical energy in a batteryconnected to the main bus; connecting a plurality of loads to the mainbus, wherein at least one of the plurality of loads is a high currentload; and connecting at least one critical load to the main bus using anelectronic boost converter, wherein the electronic boost converter isconfigured to increase voltage supplied to the critical load in responseto a voltage dip on the main bus caused by a high current transient ofthe high current load.
 18. The method of claim 17 further comprisingcontrolling electrical power supplied to the at least one critical loadusing a smart junction box.
 19. The method of claim 18 whereingenerating electrical energy comprises operating a switched reluctancegenerator.
 20. The method of claim 19 wherein generating electricalenergy comprises operating a switched reluctance generator.
 21. Themethod of claim 17 further comprising controlling the generation ofelectrical energy in response to a current required by the high currentload.
 22. The method of claim 18 further comprising controlling thegeneration of electrical energy in response to a current required by thehigh current load.
 23. An automotive electrical system comprising: amain bus; a means for storing electrical energy connected to the mainbus; a means for generating electrical energy connected to the main bus;a means for regulating the means for generating; a plurality of loadsconnected to the main bus; and at least one critical load wherein thecritical load is powered by a means for maintaining a relativelyconstant voltage supplied to the critical load.
 24. The automotiveelectrical system of claim 23 further comprising means for controllingelectric power supplied to the critical load.
 25. The automotiveelectrical system of claim 23 further comprising means for communicatingcurrent required by a high current load to the means for regulating suchthat the means for regulating controls the means for generating inresponse to the current required by the high current load.
 26. Theautomotive electrical system of claim 24 further comprising means forcommunicating current required by a high current load to the means forregulating such that the means for regulating controls the means forgenerating in response to the current required by the high current load.